2 



Dr. B. Stewart on an Apparatus [June 20, 



In accordance with the Statutes, notice of the ensuing Anniversary Meet- 

 ing for the election of Council and Officers was given from the Chair. 



. The following communication was read : — 



I, Description of an Apparatus for the Verification of Sextants 

 designed and constructed by Mr. T. Cooke^ and recently erected 

 at the Kew Observatory.^^ By Balfour Stewart, LL.D., 

 Superintendent of the Kew Observatory. Received May 9, 

 1867. 

 * 



In order to test the accuracy of graduation of a sextant, it is necessary 

 to have a series of well-defined objects, the angular distances between 

 which must be accurately known. The sextant under trial is made to 

 measure these angular distances ; and the results thus obtained, when 

 compared with the correct values of these distances (supposed to be 

 otherwise determined), will give at once the error of the instrument. 



Now with regard to this series of objects, the following conditions are 

 necessary in order that they may be convenient for the purpose of testing 

 sextants : — • 



a. It is necessary that the objects should be distinctly seen and 

 well defined. Luminous objects would be preferable, if these 

 could be obtained. Luminous points would answer well. 



/3. It is necessary that they should be at a very great, or virtually 

 infinite distance from the sextant, so that two lines proceed- 

 ing from any point in the objects, the one to the index-glass, 

 and the other to the horizon-glass, should be virtually parallel 

 to each other. 



y. It is necessary that these objects should be at such angular 

 distances from one another, that by means of them it may be 

 possible to test, say every 15° of a sextant's arc. 



I. It is necessary that these objects should be always visible, or at 

 least that they should be rendered visible easily, and without 

 loss of time. 



A series of fixed stars, at suitable intervals from one another, might 

 be made to fulfil the first three of these conditions ; but in this uncer- 

 tain climate it would be inconvenient to adopt any mode of verification 

 depending for its success upon the visibility of the sun or stars ; — in fine 

 we must have a source of light which can always be commanded. . 



A plan proposed by Mr. T. Cooke fulfils this requirement, and as it 

 has now been carried out with apparent success, a short description of 

 it may perhaps be acceptable to the Eoyal Society. 



His arrangement is of the following ilature : — 



Gr denote two collimators, F having a single vertical line, and Gr a 

 couple of cross lines, as shown in the figure. 



